Satellite communication terminals require a subsystem to track the satellites with which they communicate. This requirement exists even with stationary ground terminals and geo-stationary satellites. While tracking provides an uninterrupted link throughout a lengthy operation, it also helps in initial acquisition of the satellite.
Most existing systems either use difference patterns or step-track on the main beam. Antennas on dynamic platforms (air-borne or naval) require a faster response tracking. Sequential lobing and nutating feeds are other forms of tracking on the main beam with a higher error slope at the expense of beam offset loss. All of these “tracking on the main sum beam” schemes, also commonly called “con-scan”, become extremely inefficient in multiband antennas when tracking is done on the broader receive pattern while the narrower transmit pattern steers away from the satellite suffering an extreme pointing loss.
The difference patterns provide an error-slope for a most accurate tracking scheme with a quick response. The difference patterns in turn can either be used in a monopulse system or a pseudo-monopulse system.
When covered with one broadband device, the transmit and receive frequencies encompass a one very wide band. In the commercial C-band and Ku-bands and the military Ka-Band this bandwidth is 40% with a ratio of 2/3 between the Receive and transmit bands. In the military X-band this total receive and transmit bandwidth is relatively narrower at 12%, and in the EHF (K- and Q-bands) it is relatively wider at 81%.
When designing an antenna system that operates simultaneously over multiple bands (i.e. X- and Ka-bands), each with its separate receive and transmit bands, there may be a requirement for a composite feed with separate waveguide parts for each band nested coaxially. Conventional one waveguide port horn systems do not satisfy this requirement.
It is desirable to nest the feeds for the different bands. Except for the innermost feed, which has the smallest size waveguide operating at the highest frequency band, conventional feeds to not solve this problem. The hollowed-out outer aperture of the feed operating at the lower frequency bands requires adaptations in the designs for the orthomode transducers (OMTs), polarizers and horns. In such a nested feed, all beams are pointed at the same satellite, so it is sufficient to track in any one band at any one frequency.
In the multi-band system where the feeds are not co-located—but the aperture is partitioned into real and virtual focal points in a dual reflector system by a frequency selective surface (FSS)—, a pointing error may emerge between the two feeds. When one of the bands is at a much higher frequency, it may be mandatory to track at the higher frequency band and rely on the broader beam of the lower frequency, so as not to suffer a pointing loss. (i.e. X- and Ka-bands)
As a frequency of the band of operation gets higher and higher, the antenna beam becomes excessively narrow, and tracking stability and speed become issues with tracking on the mean beam. Such is the case in evolving Ka-band and Q-Band terminals.
When a combination of receive and transmit bands are widely separated and have to be covered separately, a dual feed system is required. This is typically the case with the EHF (K- and Q-bands). The problem is exacerbated if space is limited, and the feed has to be made compact and cannot be separated into multiple feeds employing frequency selective partitions nor partitioned into clusters.
Even in the single band of operation, some small terminals with low f/d ratios, such as ring-focus antennas, a very compact feed may be required.
Systems capable of operating over multiple bands are desirable. Know systems includes feeds or feed systems that cover widely separated bands of operation, typically in (a) multiple feed systems with frequency selective surfaces and co-located/coaxial feeds with multiple ports for multiple bands, or in (b) dual-band corrugated horns pushing the limits.
The first scheme cannot be used in compact reflector systems with small apertures and small f/d ratios because of complexity and size of waveguide runs. Most ring focus reflector systems can not employ this scheme.
In the second scheme, it is known to use nested coaxial multi-band feeds. For example, the Lincoln Labs dual band EHF feed receives in the 20 GHz K-band and transmits in the 44 GHz Q-band; and the commercial Austin Info. Sys. multi-band feed receives at 20 GHz and transmits at 44 GHz.
It is accordingly an object of the present invention to obviate many of the deficiencies of known systems and to provide a novel method and tracking feed system with multi-band operation.
This and many other objects and advantages will be readily apparent to one of skill in this art from the following detailed descriptions of referred embodiments when read in conjunction with the appended drawings.